The present invention relates to an optical communications system and more specifically to a power supply system used in an optical communications system in which at least one optical repeater is provided in an optical communications path; a regulated DC current source, which supplies a constant DC current to each repeater, is provided at one or both ends of the optical communications path; and power is supplied to each optical repeater from the DC current source through conductors.
A power supply system according to the present invention can be employed in an ordinary optical communications system and is also suitable for use in an optical communications system having a long transmitting or receiving distance requirement, for example, an underwater optical communication system.
In various wired power supply systems for communications systems, such as a power supply system which supplies power from a terminal station to intermediate repeaters provided at constant intervals along a cable or cables, a variety of methods are known to supply regulated AC or DC voltage or current to the repeaters, the repeaters being connected in series or in parallel throughout the power supply path.
If AC power is supplied to the repeaters, rectifying and smoothing circuits must be provided for the repeaters, resulting in a complicated circuit. An advantage of using AC power, however is that a simple transformer can be employed to supply the AC voltage. This power supply method is often employed in transmission systems utilizing electron tube type repeaters, which require various voltage values to function. However, the power factor is lowered by the capacitance and inductance of the transmission cables used and, therefore, the voltage drop is high, reducing the power transmission efficiency (power required for all repeaters .div. power transmitted).
Since the output of DC power supplies is not influenced as greatly by the inductance and capacitance of the transmission cable, it is relatively easy to supply constant power to the repeaters utilizing a DC power supply, and still maintain high power transmission efficiency. Accordingly, this method is often employed in transmission systems utilizing transistorized repeaters which are operated by DC voltage. In addition, since an underwater coaxial communications system requires highly reliable repeaters and must provide signal power transmission over a long distance, a DC power supply system, having a very stable output and utilizing repeaters of simple construction, is employed.
FIG. 1 illustrates an ordinary DC power supply system for a communication system. The n-repeaters, 1 through n respectively, are connected in series to the power supply path C1, and a constant current I is supplied to each repeater from the regulated current supply equipment 1 and 10, respectively provided at the end of the power transmission path. In this case, the regulated current supply equipment 1 and 10 is provided at both ends because each power supply provides power to n/2 repeaters and, as a result, the load on the power supplies is reduced. This method is used for a system which has a long transmission distance requirement, such as an underwater coaxial communication system. If the transmission distance is short, or if each power supply in FIG. 1 has a sufficient power rating and one of the power supplies fails, power can be supplied by only one power supply. The DC power supply system illustrated in FIG. 1 can also be employed in an optical fiber cable communication system, including an underwater optical communication system.
In an optical transmission system it is impossible to supply the power supply current by superposing it on the transmission signal using the communication path itself as is done in a coaxial system, so a power transmission conducting path is provided within the optical fiber cable, and power is fed through the conducting path. This cable arrangement is illustrated in FIG. 3, which is described below.
FIG. 2 illustrates an optical repeater used in an existing optical communication system. As illustrated in Fig. 2, an optical repeater has a signal amplifier circuit 42 comprising a photoelectric conversion element, an amplifier circuit and a light-emitting element. A current I" is drawn from the supply current I' provided through the power supply path Cl, in order to supply the necessary power to the signal amplifier circuit 42. A voltage V.sub.R across zener diode 41', obtained by applying current I'" to the zener diode 41', is also supplied to the signal amplifier circuit 42. As a result, a power V.sub.R .times.I" is supplied to the signal amplifying circuit 42. C2 is a signal transmission path formed by an optical fiber. The conventional power supply circuit 41' illustrated in FIG. 2 is formed by a zener diode and has the advantage that it is small and reliable, but it also has the disadvantage that it raises the supply current requirement of the optical repeater to about 1 to 2A. This value is quite a bit larger than the supply current requirement for a coaxial cable system, which is 100 to 200mA. The difference in requirements is due to the fact that an optical repeater requires a bias current and a drive current for an optical semiconductor, such as a laser diode or an avalanche photodiode, and therefore a larger amount of current is consumed by an optical repeater system in comparison to a coaxial cable system.
Since the power consumed by the power transmission path C1 of FIG. 2 is proportional to the square of the supply current in a regulated current supply system, the power supply equipment in a conventional optical communication system must be considerably large in size in order to compensate for the power consumption in the power transmission path C1. This is particularly true in a long range transmission system such as an underwater optical communication system. This large power consumption by the power transmission path C1 is a serious problem from an economic standpoint. For example, in a conventional coaxial cable system, a supply voltage of approximately 5kV is required, but in a conventional optical communication system, a supply voltage of approximately 15kV is required. As a result of having to supply such a high voltage, the selection of circuit elements and mounting of such circuit elements becomes difficult and expensive.